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AAPG ACE 2018

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Petrology of the Hydrothermal and Evaporitic Continental Cretaceous (Aptian) Pre-Salt Carbonates and Associated Rocks, South Atlantic Santos Basin, Offshore Brazil

Abstract

Continental carbonate and non-carbonate rocks associated with coeval basalts comprise a unique Cretaceous (Aptian) sag pre-salt succession in Santos Basin, offshore Brazil. The autochthonous sedimentary rocks were formed by a combination of abiogenic and biogenic processes. The abiogenic group is predominant and includes crystal-shrub limestones, bedded travertines and tufas, and Mg-rich silicate argillites. The crystal-shrub limestones are composed of fascicular-optic calcite aggregates with sweeping extinction and irregular upward divergent branching pattern. The bedded travertines are characterized by centimetric horizontal or inclined beds composed by microcrystalline and pseudo-peloidal calcite with fenestral porosity. Tufas are represented by incrusted organic undefined structures. Mg-rich silicate argillites mainly composed by kerolite, stevensite, MLM kerolite/stevensite, and sepiolite occur intercalated with the carbonate rocks. Petrographic, geochemical (isotope and elemental), and fluid inclusion data suggest formation in a low-temperature hydrothermal and evaporitic environment, with strong influence of ascending focalized Ca-Mg-Si rich fluids emerging in subaerial and sub lacustrine springs. The biogenic rocks are represented by laminated microbial boundstones.

Calcrete, dolocrete, and silcrete are diagenetic facies that occur as distinct layers throughout the entire sequence. Spherulitic and shruby calcitic nodulization and intense Mg-clay replacement by discrete zoned dolomite are the most common diagenetic features of phreatic argillaceous calcretes and dolocretes.

Silcretes and hydraulic breccias are composed by banded chalcedony, micro, and macroquartz, which occur both replacing and cementing primary constituents. Dawsonite, celestite, barite, saddle dolomite, and fluorite are volumetrically minor diagenetic components.

The allochthonous rocks are carbonate rudstones and grainstones, hybrid conglomerates, arenites and lutites, mostly composed by intrabasinal particles derived from the erosion of the lithotypes described above, which indicates a strong coeval cannibalization of the system. Fossils are scarce in the entire succession.

Similar analog deposits occurs in Lake Bogoria, Afar rift, Mono Lake, Pyramid Lake, Yellowstone Park, Denizli-Pamukale, Tivoli, and San Juan travertine systems, and Madrid Basin, although none of them present the whole spectrum of deposits identified within the Aptian pre-salt succession in Santos Basin.